A known form of apparatus for assessing a property of a fluid comprises an optical fiber coated near its tip with an enzyme or antibody. When the coated tip is contacted with the fluid, enzyme-catalyzed reactions create localized heating and expansion of the fiber and the rate of expansion, ana hence the rate of reaction, is measured by comparing the fiber length with that of an adjacent reference fiber using interferometry. The known apparatus is not temperature stable and operation depends on the expansion of an optical fiber which is a disadvantage where space is limited.